1. Field of the Invention
The present invention relates to photovoltaic power generation systems and methods of controlling such photovoltaic power generation systems. More particularly, the present invention concerns photovoltaic power generation systems that are constructed to prevent an intruder, who is attempting to intrude into a restricted area including an area in which solar cells are installed, from receiving an electric shock, and methods of controlling such photovoltaic power generation systems.
2. Related Background Art
In recent years, awareness of environments is spreading across the world. Among others, serious concerns are exhaustion of fossil fuels and the like and global warming due to CO2 emissions, so that the desire for clean energy is growing increasingly. Attention is thus being directed toward photovoltaic energy, which is clean energy resulting from direct conversion of solar energy to electric power, and solar cells are quickly spreading in use in various consumer-oriented fields.
In general solar cell modules possess sufficient electric insulation and enough safety to prevent an electric shock hazard even if a man should touch them. The solar cell modules are thus commercially available and are beginning to spread, but the solar cell modules involve an issue of still high cost, because a lot of coatings and insulators need to be used for ensuring the safety. Consequently, they are still unsatisfactory for application of solar cells high in power generation cost to power plants and thus are not used widely. On the other hand, there is a danger that an electric shock hazard may occur when a person except for a manager intrudes into a hazard controlled area in the power plants and the like, and it can extend to an accident in the electric power system. For eliminating this risk, studies have been made heretofore on various intrusion detecting devices and safety devices.
For example, an intruder detecting device described in Japanese Patent Publication No. 63-67718 is an intrusion detecting device composed of an outer detector placed around facilities, though not a power generation plant, and functioning to detect a passing object, an inner detector placed between the outer detector and the facilities and around the facilities and functioning to detect a passing object, an outer detecting circuit and an inner detecting circuit each having a filter for passing only frequencies caused by passage of a manager or a person concerned, among output of the outer and inner detectors, and a level detector for detecting only vibration levels due to a man, and an intrusion detecting circuit for counting output of the outer or inner detecting circuit and producing an intrusion detection signal when a count reaches a predetermined value within a given time.
Conventionally, studies were conducted on methods of preventing intrusion of an intruder or on methods of detecting an intruder by making use of the monitoring method as described above. In the conventional power plants such as thermal power plants, atomic power plants, and so on, however, if the system is brought once to a halt, a considerable time will be required for a restart of the system and thus the halt will cause great disadvantage. Therefore, there was no conception that the entire power generation system was brought to a halt as a safeguard against intruders and, simply, an alarm or the like was issued to let guards notice.
The photovoltaic power generation systems with solar cells installed on the roofs of general houses have become widespread in these years.
In such photovoltaic power generation systems, solar cell modules are connected in series to obtain a desired voltage. This series of solar cell modules is generally called a string. Several strings are connected in parallel to obtain a desired electric current, thus constructing a solar cell array with a power generation scale of desired value. This solar cell array is equipped with components for control and connection of peripheral equipment, thereby composing a photovoltaic power generation system. Accordingly, the solar cell array becomes larger with increase in the power generation scale required of the photovoltaic power generation system, and the number of solar cell modules constituting it also increases. Therefore, in designing of a photovoltaic power generation system, the system is formed by connecting the predetermined number of modules or solar cells of a predetermined area in series and in parallel so as to realize predetermined output voltage and current and a predetermined power generation capacity.
It is feasible to implement power generation with a greater capacity, by increasing the number of series/parallel solar cell modules. However, as the power generation capacity increases, the voltage and current also become larger with increase in the number of series/parallel modules. In general the power generation capability of solar cells is determined by intensity of sunlight incident thereto. Describing it in brief, the photovoltaic power generation system operating at a fixed voltage varies its operating current with variation in insolation (solar radiation) to determine its output power.
The photovoltaic power generation system and the method of protecting the power generation system, described in Japanese Patent Application Laid-Open No. 7-177652, are configured to have a first switch for effecting connection or disconnection between positive and negative power terminals of a solar cell string and output lines connected in parallel thereto and outputting power, a second switch for establishing a short circuit or an open circuit between the power output terminals, and a controller for comparing a voltage value between detection terminals at a predetermined position of the solar cell string, with a predetermined value and controlling switching of the first switch and the second switch, based on the comparison signal. In the photovoltaic power generation system and the method of protecting the photovoltaic power generation system, the solar cell string is judged abnormal when the voltage value between the detection terminals thereof becomes not more than the predetermined value, the power output terminals of this solar cell string are then disconnected from the output lines, and a short circuit is established between the power output terminals, thereby isolating and deactivating the defective solar cell string from the photovoltaic power generation system.
However, in the case of the power plants using the solar cell modules, since the installation scale was large and a large area was necessary, it was difficult to prevent the electric shock hazard only by the conventional means of simply giving an alarm or the like to let the guards note the intrusion of intruder and the number of guards was preferably as small as possible from the aspect of cost. Therefore, the conventional means had a limit.
The solar cell modules used in the photovoltaic power generation systems are usually designed so as to ensure the electrical insulation performance in normal use in the initial stage and after installation. It cannot be denied, however, that there is a possibility of degradation or failure of electrical insulation due to unpredictable causes after installation. There can also occur failure of electrical insulation, for example, due to breakage of the coating over the surface of the solar cell modules, cracks, scars, deformation, etc. of surface glass, and so on for some reason. No one can deny that there is a risk of an electric shock or the like when a man comes to near an exposed portion or a defective portion appearing unpredictably and touches it. The risk of electric shock is high with failure of electrical insulation, particularly, in the case of the large power generation scales, because the voltage and current also become large with increase in the number of series and parallel solar cells, as described previously.
In addition, the solar cell arrays are often installed relatively near the ground with the recent proliferation of the photovoltaic power generation systems, and there is an increasing possibility that a child or a worker not associated with the photovoltaic power generation apparatus (photovoltaic power generation system) touches the solar cell array. It is thus also becoming important to give consideration to prevention of an electric shock. It is the present status that the solar cell array is located within a secure fence and the solar cell array is not allowed to be located at a place which a lot of people go into and out of.
The photovoltaic power generation system and the protection method of the photovoltaic power generation system described in No. 7-177652 were configured to disconnect the solar cell string upon occurrence of disadvantage and no consideration was given to the safeguard against intruders. No consideration was given to malfunctions of the protection system, e.g., in the case of weak insolation, either.